In Computer Science class we have a course about robotics and we where given the task to build a walking robot using Lego Mindstorms (with a 2 hour time frame).

Due to the limited amount of parts available we settled on a 4 legged design. However after 2 hours of struggle we went home empty-handed. Being annoyed that it didn’t work out, I continued at home to build a simpler Technic model. Two hours later I had this:

4-legged walker

[youtube=http://www.youtube.com/watch?v=GCmyogVerC0]
(Sorry for the bad lightning)

Balance showed to be crucial to the walking performance, the two round 2×2 bricks sole purpose is to balance the walker as without them it became (even more) unstable. It was probably also the issue with the failed NXT model as it had a lot more weight and fell over all the time. (The motor speed might have been too high too though.)
Interestingly enough, the surface had an effect on how the model should be balanced.

6-legged walker

I have previously build another small walker similar to the above but with 6 legs. It showed to be a much better solution than the 4-legged version as the stability is much better.

[youtube=http://www.youtube.com/watch?v=KpDoNH0Ibnc]

There is also a CAD file, I will upload it to brickshelf or something later…

Bonus

Made this for fun, a motor and battery box is inside making it able to drive. It is not that great though since the motor connection is pretty weak.